1. Technical Field of the Invention
The embodiments of the invention relate to VCOs and, more particularly, to temperature compensation in the operation of VCOs.
2. Description of Related Art
Various wireless communication systems are known today to provide links between devices, whether directly or through a network. Such communication systems range from national and/or international cellular telephone systems, the Internet, point-to-point in-home system, as well as other systems. Communication systems typically operate in accordance with one or more communication standards or protocol. For instance, wireless communication systems may operate using protocols, such as IEEE 802.11, Bluetooth™, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), as well as others.
For each wireless communication device to participate in wireless communications, it generally includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, modem, etc.). Typically, the transceiver includes a baseband processing stage and a radio frequency (RF) stage. The baseband processing provides the conversion from data to baseband signals for transmitting and baseband signals to data for receiving, in accordance with a particular wireless communication protocol. The baseband processing stage is coupled to a RF stage (transmitter section and receiver section) that provides the conversion between the baseband signals and RF signals. The RF stage may be a direct conversion transceiver that converts directly between baseband and RF or may include one or more intermediate frequency stage(s).
Furthermore, wireless devices typically operate within certain radio frequency ranges or band established by one or more communications standards or protocols. A local oscillator generally provides a local oscillation signal that is used to mix with received RF signals or baseband signals that are to be converted to RF signals in the modulation/demodulation stage of the RF front end. A synthesizer may be used to set the frequencies to drive the local oscillator to provide the desired frequencies for mixing, in which the desired frequencies are generally based on the channel frequencies established for the particular standard or protocol.
To generate various reference signals, clock signals, channel frequencies, etc., a wireless device typically uses a phase locked loop (PLL) circuit to produce a signal that locks to a particular frequency. Furthermore, in a typical (PLL), a control voltage is input to a voltage controlled oscillator (VCO), in which the control voltage establishes the frequency output from the VCO. Accordingly, for stable performance, the VCO should generate and maintain a locked frequency for a selected control voltage input.
However, the operational characteristics of the VCO and the PLL may vary if significant temperature variations are encountered by a device. That is, the VCO may experience frequency drift at its output, if temperature varies appreciably. For example, in continuous operation of a cellular 3G or 4G (3rd generation or 4th generation) system, a VCO frequency drift over temperature may cause the PLL to unlock due to the VCO's inability to recalibrate over a temperature change. This condition may result, when the control voltage of the VCO exceeds the designated specification limits for the VCO, due to the temperature change. This problem becomes more pronounced at lower supply voltages, such as at supply voltages of approximately 1.2 volts, or less. In such a situation, a 3G wireless VCO may have 50 MHz drift over an operating temperature range of −20° C. to +85° C.
Accordingly, there is a need to obtain a much more stable VCO when operating at lower supply voltages over an appreciable temperature range in order to reduce or inhibit frequency drift of the PLL.